In recent years, lithium secondary batteries typified by a lithium secondary battery having high energy density and good cycle performance and scarce self-discharge have drawn attention as power sources for portable equipments such as mobile phones, notebook personal computers, etc. and electric automobiles.
Lithium secondary batteries presently in the main stream are those with 2 Ah or lower in compact sizes for consumer uses, mainly for mobile phones. As a positive active material for a lithium secondary battery, many of positive active materials have been proposed and most commonly known materials are lithium-containing transition metal oxides operating voltage around 4 V containing, as a basic configuration, such as a lithium cobalt oxide (LiCoO2) with an, a lithium nickel oxide (LiNiO2), or a lithium manganese oxide (LiMn2O4) having a spinel structure. Especially, since the lithium cobalt oxide has excellent charge and discharge performance and energy density, it has been employed widely as a positive active material for a lithium secondary battery with a capacity as low as 2 Ah battery capacity.
However, in the case of consideration of developing nonaqueous electrolyte batteries for industrial uses, which are highly expected to become in a middle to large scale, particularly a strong demand in the future, the safety would be considered to be very important and it can be said that the present specification of compact batteries is not necessarily sufficient. One of the reasons is thermal instability of positive active materials and various countermeasures have been taken; however they have not been yet sufficient. Further, it needs to be assumed that, in industrial uses, batteries would be used in high temperature environments in which the batteries in compact sizes for consumer uses would not be used. In such high temperature environments, not only conventional lithium secondary batteries but also nickel-cadmium batteries and lead-acid batteries have also very short lives and presently, there are no batteries that satisfy the requests of users among conventional ones. Capacitors are merely those which can be used in the temperature range; however, the capacitors have small energy density and in this point, they fail to satisfy the requests of users and accordingly, batteries with a long life in a high temperature and high energy density have been desired.
Recently, a polyanion-based active material excellent in heat stability has drawn attention. Since oxygen in the polyanion-based active material is fixed by covalent bonding with an element other than transition metals, the polyanion-based active material emits no oxygen even at a high temperature and it is considered that the safety of a battery can be remarkably improved by being used as an active material.
Investigations of lithium iron phosphate (LiFePO4) having an olivine structure as such a polyanion-based positive active material have been actively carried out. Since having a theoretical capacity as high as 170 mAh/g and being capable of inserting and extracting lithium at high potential 3.4 V (vs. Li/Li+), lithium iron phosphate has energy density as high as that of LiCoO2 and has been expected large as a positive active material for replacing LiCoO2.
Some trials for synthesis of partially substituted lithium iron phosphate by cobalt for iron have been reported.
Patent Document 1 describes synthesis of LiCo0.25Fe0.75PO4, LiCo0.5Fe0.5PO4, LiCo0.75Fe0.25PO4, and LiCoPO4, which are equivalent to the amounts of substituted by Co of 25%, 50%, 75%, and 100%, by a solid-phase method, production of nonaqueous electrolyte secondary batteries by using these compounds as positive active materials and lithium metal as a negative electrode; and results of evaluation of discharge capacities after charging to 5.3 V and discloses that “those with a higher cobalt content show a higher discharge voltage and a voltage exceeding 4.5 V in the plateau part of the discharge voltage” (in paragraph 0022) and that the discharge capacity along with Co amount is increased or decreased in accordance with a discharge end voltage (Table 1).
Non-patent Document 1 describes synthesis of LiFe0.8Co0.2PO4, LiFe0.5Co0.5PO4, and LiFe0.2Co0.8PO4, which are equivalent to the amounts of substitution by Co of 20%, 50%, and 80%, by a solid-phase method, production of single electrode evaluation cells by using these compounds as positive active materials, and results of charge and discharge cycle tests at a charging potential of 5 V and results of charge and discharge cycle tests at a charging potential of 4V using LiFePO4 as a positive active material and discloses that in the case of 20% with substitution by Co, the initial discharge capacity is slightly increased as compared with that in the case of using LiFePO4; however the capacity retention ratio along with the charge and discharge cycle is worsened as compared with that in the case of using LiFePO4; and that in the case of 50% and 80% with substitution by Co, the initial discharge capacity and also the capacity retention ratio along with the charge and discharge cycle are both considerably worsened as compared with those in the case of using LiFePO4 (FIG. 2).
Non-patent Document 1 describes synthesis of LiFe0.9Co0.1PO4, which is equivalent to the amount of substitution by Co of 10%, by a solid-phase method, production of nonaqueous electrolyte secondary batteries by using the compound as a positive active material and lithium metal as a negative electrode, and results of comparison of the discharge end capacity at 2.0 V after charging to 4.5 V with that of LiFePO4 and discloses that in high rate discharge performance at 5 C discharge is improved; however the discharge capacity at 0.2 C discharge is lowered (see FIG. 3).
Non-patent Document 3 describes synthesis of LiFe1-xCoxPO4 (x=0.02, 0.04, 0.08, and 0.1), which are equivalent to the amounts of substitution by Co of 2%, 4%, 8% and 10%, production of nonaqueous electrolyte secondary batteries by using these compounds as positive active material and metals lithium for a negative electrode, conduct of an initial discharge test and a charge and discharge cycle test at a charging voltage of 5.1 V, and results of comparison with those in the case of using LiFePO4 at a charging voltage of 4.25 V and discloses in FIG. 7 that in all of batteries using the compounds substituted by Co, the discharge capacity is decreased as compared with that in the case of using LiFePO4 and discloses in Abstract that substitution by Co causes an adverse effect on the electrochemical performance of lithium iron phosphate. Further, it is also disclosed that a charge and discharge cycle test is carried out at 1 C discharge ratio (equivalent to 1 It), and as a result, no more than about 50% of capacity is obtained (FIG. 8).
In Non-patent Document 3, the above-mentioned various positive active materials are synthesized by dissolving LiOH.H2O and FeC2O4.2H2O in nitric acid, dropwise adding a (NH4)H2PO4 solution together with citric acid to the obtained solution, heating the mixture at 75° C., and drying the obtained gel at 110° C. to obtain precursors which are then calcined at 750° C. for 10 hours in Ar atmosphere and successively at 850° C. for 2 hours. There is a description that regardless of presence or absence of substitution by Co, existence of a small amount of Fe2P impurity phase is confirmed in all of the synthesized samples by X-ray diffraction patterns. Regarding this point, Non-patent Document 4 demonstrates that the improvement of electron conductivity is not attributed to doping with a small amount of different elements but existence of a phosphide phase such as phase separated Fe2P formed by mixing such different elements and carrying out the synthesis contributes to the improvement of electron conductivity from the results of measurements by a transmission electron microscope (TEM) and an electron energy-loss spectroscopy (EELS) in combination.
It is known that the redox reaction generated alone with electrochemical insertion and extraction of lithium in and from LiFePO4 proceeds at a relatively low potential around 3.4 V (vs. Li/Li+) and on the other hand, the reaction proceeds at a relatively high potential around 4.8 V (vs. Li/Li+) in the case of LiCoPO4. In general, partially substituted LiFePO4 by Co for Fe aims to give high energy density as a positive active material by utilizing high redox potential of LiCoPO4. Therefore, in the conventional technical documents described above, the battery performance is evaluated by employing potentials sufficiently high for changing the valence of Co by charging.
However, in lithium secondary batteries aiming for actual industrial uses, designing needs to be carried out in consideration of oxidation resistance issue of a nonaqueous electrolyte solution and therefore, charging at a positive electrode potential exceeding 4.2 V causes a problem in terms of battery performance.
Accordingly, as a prior condition of the invention aiming for industrial uses, various kinds of issues need to be solved on condition that the positive electrode potential does not reach over 4.2 V at the time of charging while the characteristic which LiFePO4 intrinsically has such that the insertion and extraction reaction of lithium proceeds at a relatively low potential is utilized. Herein, it can be said that the invention relates to a lithium secondary battery to be used in a range where a positive electrode potential does not exceed 4.2 V. In examples described below, the positive electrode potential is made to reach 3.8 V or 3.6 V at the time of charging, excluding the charge and discharge test, high rate discharge test, and charge and discharge cycle test in Examples 5 to 8 and Comparative Examples 6 to 10.
In consideration of the fact that the redox potential of LiCoPO4 is around 4.8V (vs. Li/Li+) while putting aside the fact that many documents report partial substitution by Co for Fe of LiFePO4 lowers the discharge capacity, even if a phenomenon such that the energy density and the like increase by the partial substitution by Co for Fe of LiFePO4 occurs on condition that charging is carried out to a potential as sufficiently high as, for example, 5 V, the phenomenon is not necessarily unrealizable. However, on condition that the positive electrode potential does not reach over 4.2 V at the time of charging, since the valence of Co cannot be changed and therefore, making Co exist in LiFePO4 is expected to result in merely deterioration of the battery performance such as reversible discharge capacity.
Further, any of the patent document and non-patent documents does not describe how the high temperature storage stability of a battery using partially substituted by Co for Fe of LiFePO4 for the positive active material would be.
Further, Patent Document 1 describes that “those usable as a negative active material, besides lithium, are a lithium alloy, a lithium compound as well as conventionally known alkali metals and alkaline earth metals such as sodium, potassium and magnesium, and substances which can absorb and desorb alkali metal and alkaline earth metal ions such as alloys of these metals and carbon materials (paragraph 0005) and also describes with respect to carbon materials to be used as the negative active material in one line. However, any of the patent document and non-patent documents neither describes batteries using partially substituted by Co for Fe of LiFePO4 for the positive active material and a carbon material for the negative electrode nor describes nor indicates how the battery performance (remaining capacity ratio and recovery capacity ratio) and the charge and discharge cycle performance would be after storage of a battery using partially substituted by Co for Fe of LiFePO4 for the positive active material and a carbon material for the negative electrode.
Further, the description how the remaining capacity ratio and the recovery capacity ratio would be in the case of using a carbon material capable of insertion and extraction a lithium ion for the negative electrode is not at all expected from the patent document and non-patent documents which only describe batteries using lithium metal for the negative electrode. It is because in the case of using the carbon material for the negative electrode, the remaining capacity ratio and the recovery capacity ratio are evaluation of the ratio of Li to be returned from the carbon material of the negative electrode at the time of discharge after a portion of Li which exists in the positive active material reaches the carbon material of the negative electrode by charging and successively the battery is stored.
Furthermore, the description how the charge and discharge performance would be in the case of using a carbon material capable of insertion and extraction a lithium ion for the negative electrode is not at all expected from the patent document and non-patent documents which only describe batteries using lithium metal for the negative electrode. It is because in the case of using lithium metal for the negative electrode, the negative electrode generally has excess capacity and the batteries become those having capacity balance limited in the positive electrode whereas in the case of using the carbon material for the negative electrode, the lithium source is all supplied from the positive electrode side and the capacity balance is generally limited in the negative electrode and accordingly, the charge and discharge cycle performance would be largely affected by the performance of the negative electrode.